1. Field of the Invention
The present invention relates to an LED array exposure device for use in recording an image, a controlling method thereof, and an image forming apparatus using the LED array exposure device.
2. Description of the Prior Art
As a device for exposing a photoconductor with digital image information, there is a laser exposure device using laser beams for exposing or an LED array exposure device using a plurality of extremely small LEDs (light-emitting diodes), each of which corresponds to one dot of the digital image, is laid linearly in an array shape, and is arranged along an axis of the photoconductor (a direction of main scanning) for exposing the photoconductor. Particularly in recent years, the LED array exposure device has been extensively used for printers and other image forming apparatuses on the grounds that the LED array exposure device is smaller in size, lower in cost, easier to control, higher in reliability because no mechanical moving parts and the like are used.
The LED array exposure device as mentioned above comprises a printed circuit board, an LED array chip mounted thereon, a driver IC for delivering current and driving the LED array chip, a lens array formed as a cluster by a plurality of lenses and arranged between a light-emitting plane of the LED array chip and the photoconductor so that light beams emitted from light-emitting elements (light-emitting diodes) are converged on the photoconductor to form an image thereon, supporting members for supporting these components, and the like.
One or a plurality of LED array chips are laid on the printed circuit board for exposing an entire effective scanning width, at least a width of a paper or wider, and constitute an exposing light source for forming an electrostatic latent image on the electrostatically charged photoconductor. Extremely small light-emitting elements, each of which corresponding to each of pixels of video data (image data) in a direction of main scanning, are aligned in a raw on the LED array chip. At least a total number of 5120 light-emitting elements are required for one or a plurality of LED array chips, when, for example, an A4 size recording width is to be exposed in a resolution of 600 dpi.
The driver IC has a circuit for driving each light-emitting element so that the light-emitting element emits light, and one or a plurality driver ICs are mounted on the printed circuit board (or provided externally). The lens array is made by arranging a plurality of cylindrically shaped lenses in a cluster form so as to converge light beams emitted from the light-emitting elements on the photoconductor so that the photoconductor is exposed by a dot.
However, the light intensity varies among the light-emitting elements. As a result, these variations show up as uneven densities or streaks in a visible image formed on a paper and cause a quality deterioration in a recorded image. To cope with this problem, a conventional LED array exposure device is provided with prearranged light intensity correction data for each light-emitting element for compensating for the intensity of light to be emitted therefrom so that exposure energy emitted from each light-emitting element is corrected in accordance with the light intensity correction data so that the exposing energy becomes equal to each other
Furthermore, when there are variations in resolving power due to an uneven layout of the lens array or when the light-emitting element is out of place and defocused due to an error in mounting the lens array, a dot shape formed on the photoconductor is deformed or the resolving power of each light-emitting element becomes uneven. Even if variations in light intensity among the light-emitting elements are corrected within ±2%, if there are variations in resolving power because of the lens array, the uneven densities will appear conspicuously in a visible image.
To overcome the above-mentioned problem, Japanese Patent Application Laid-Open No. 2002-67372 discloses an LED print head. In this LED print head, light from each light-emitting element is received and measured by a sensor portion, a relationship between the light and a scanning distance of the sensor portion is obtained, and light output of the light-emitting element is adjusted so that the intensity of the received light when sensitivity of a photoconductor is made to be threshold becomes a desired value. When a method in which the light output as measured above is converted into a theoretical beam diameter based on the threshold which is converted from the sensitivity of the photoconductor and the light output is adjusted accordingly so as to make the beam diameters uniform is employed, each beam diameter becomes equal to each other theoretically.
However, if a granularity of the visible image is large, then uneven densities or streaks become conspicuous in the image. It has been shown that the magnitude of the granularity of image varies depending on such factors as a screen angle of pixel, a sensitivity of photoconductor, a surface temperature of photoconductor (the temperature that affects the sensitivity of photoconductor), a developing bias voltage, or the like, which will be discussed later.
In a color image forming apparatus that forms a full-color image by using three or four colors, the screen angle of the image for each color is different from each other. When a LED array exposure device is used in this type of image forming apparatus, the uneven densities or streaks become conspicuous due to the screen angles that are different from color to color. As a result, not only do uneven densities or streaks occur, but also color reproducibility is adversely affected and recording quality is significantly degraded. A black and white image forming apparatus that usually uses a screen angle of 45 degrees also suffers a similar degradation in recording quality.
Furthermore, when the sensitivity of photoconductor is high, the photoconductor responds more sensitively to variations in light quantity and in beam spot area of the light-emitting element. This makes the uneven densities or streaks to be easily recognizable as if the variations were amplified. Particularly, in a tandem-type color image forming apparatus that uses a plurality of image forming sections for forming an image in different colors simultaneously, the photoconductors on which the image is formed are different. Because of this reason, unless variations in sensitivity are compensated, the magnitude of the uneven densities becomes different from color to color, thereby causing an adverse effect in reproducibility of colors. Moreover, depending on types, there are such photoconductors of which the sensitivity is largely influenced by the surface temperature thereof. This means that, if the photoconductor is highly temperature dependant, the same problem is caused by fluctuations of the surface temperature of the photoconductor.
It has also been shown that the uneven densities tend to become conspicuous depending on the developing bias voltage to be applied to a developing roller during developing process in which a latent image is developed and made visible. Because the developing bias voltage acts as a so-called threshold value when the electrostatically charged latent image attracts toners, a higher bias voltage raises the threshold value for a high-density pixel and a lower bias voltage lowers the threshold value for a low-density pixel respectively. As a result of this, in either case, the variations in light quantity and in beam spot area among the light-emitting elements are reflected conspicuously so as to be easily recognized as uneven densities or streaks. Particularly, because properties of toners or developing units are different from color to color in the tandem-type color image forming apparatus, the bias voltage to be applied to each of the developing units may be set differently from each other. Moreover, even in a black and white image forming apparatus having a single developing unit, it is sometimes necessary to adjust and set a different bias voltage from apparatus to apparatus due to variations among components and apparatuses. In this case, the influence of different bias voltages exerted upon the occurrence of uneven densities and streaks cannot be ignored.
However, the aforementioned conventional technology does not suggest performing a control for correcting for factors that aggravate the above-mentioned granularity of image.